1. Field of the Invention
The present invention relates to an improvement of a thermoelectric material useful for power generation, refrigeration, and the like, and also to a sensor using the thermoelectric material. More specifically, the invention relates to a thermoelectric material using a material having a Seebeck effect and a sensor using the same thermoelectric material, and to a method of manufacturing the same.
2. Description of the Prior Art
The Seebeck effect is a phenomenon wherein, when a temperature difference arises between two different points of an electrically conductive material, an electromotive force is developed in proportion to the temperature difference. The electromotive force consists of two components, i.e., an electromotive force due to distribution of charge carrier density caused by a migration of charge carriers for keeping constant free energy of the charge carriers within the material, and another electromotive force due to an interaction between charge carriers and heat flow, namely, phonon flow from higher- to lower-temperature portions.
These charge carriers and phonons behave differently between a surface portion (up to about 100 .ANG. in depth) and an internal portion of the material. Therefore, thermoelectric effects of the material such as thermal electromotive force (i.e., Seebeck electromotive force) are different between its surface portion and internal portion. For thin films having a thickness on the order up to 1 .mu.m or fine particles having a diameter on the order up to 1 .mu.m, the surface portion (interface portion) occupies larger part relative to the internal portion so that the thermoelectric effect in the surface portion can no longer be an ignorable phenomenon.
Generally, a surface of a thermoelectric material has atmospheric gasses such as oxygen, nitrogen, steam, carbon dioxide and the like adsorbed thereon, which, it is considered, would affect the thermoelectric effects on the surface portion (interface portion). Obeying Langmuir's adsorption isotherm, an adsorption amount of atmospheric gas increases with increasing the pressure of the atmospheric gas so that the higher the pressure of the atmospheric gas, the greater the change in the thermal electromotive force.
Thermoelectric materials capable of interconversion between temperature difference and electromotive force have been manufactured heretofore, as bulk materials, by melt-casting a thermoelectric material or calcining a powdery material at a high temperature, and their resulting fine structures are shown in FIGS. 7 and 8, respectively.
Referring to FIGS. 7 and 8, in the conventional material prepared by melt-casting a thermoelectric material, crystal grains are arranged densely and continuously although a small amount of cracks 13 and pores 14 may exist as shown in FIG. 7. In the conventional powder product calcined at a high temperature, the particles are partly fused and electrically linked to each other as shown by reference numeral 15 in FIG. 8. On the other hand, there can be obtained thin-film type thermoelectric materials formed through deposition on a glass substrate or organic membrane or other methods. In either case of the conventional methods, thermoelectric materials have been provided in a structure as dense as possible so that their electrical conductivity and mechanical strength as a bulk material would be enhanced for increased practicability.
Sensors using such thermoelectric materials are in most cases implemented by making use of a function of detecting temperature difference. Examples of such sensors include temperature sensors or electric thermometers incorporating alloy thermocouples such as iron/gold--chromel and alumel--chromel thermocouples. An example of the sensors can be found in an application for detecting an extinction of a pilot burner of a boiler by sensing a change in temperature difference.
Although there have already been developed semiconductor sensors using changes in electric resistance and sensors using enzyme electrode reactions as gas-pressure sensors or specified-substance oriented sensors, no sensors have been proposed yet to which any thermoelectric phenomenon has been applied.
Conventional thermoelectric materials have been used as materials for converting temperature difference into electricity due to thermal electromotive force or obtaining temperature difference by conducting an electric current through the materials. As for sensors using these materials, which have been applied only for detecting temperature difference, there is a difficulty in measuring other physical quantities such as gas pressure, concentration of a substance in a solution.